This invention relates generally to a mechanism for preventing the skew of a cut paper sheet, and more particularly to such a skew prevention mechanism used in a duplex printing machine and a paper reversing device.
FIG. 6 is a schematic side-elevational view of a duplex printing laser printer, and a conventional duplex printing laser printer will now be described with reference to FIG. 6. A photosensitive drum 1 is electrified by an electrifier 2, and a latent image is formed on the photosensitive drum 1 by a laser light 3. A toner 12 is deposited on the latent image by a developing device 4, so that the latent image is converted into a visible image. A paper sheet 6 fed by a paper supply roller 5 is fed to a transfer portion of the photosensitive drum 1 by feed rolls 7, and the toner image on the photosensitive drum 1 is transferred to the paper sheet by a transfer corotron 8. After the transfer, the static charge is eliminated from the sheet 6 by a separating corotron 9, and the sheet 6 is separated from the photosensitive drum 1. Thereafter, the sheet 6 is conveyed by a conveyer belt 10 to a fixing device 11 where the transferred image on the sheet 6 is fixed. The residual toner, which has not been transferred to the sheet 6 and hence remains on the photosensitive drum 1, is removed from the photosensitive drum 1 by a cleaner 13 and is recovered.
The sheet 6 subjected to the printing at one side thereof is discharged onto a reverse plate 15 by paper discharge rollers 14. Then, the sheet 6 is again fed into the machine from the its trailing end by reverse rollers 16 (a conventional switchback mechanism) which are rotating in contact with one row of the paper discharge rollers 14. Immediately after the reverse roller 16, there is provided a paper path-switching gate 17, and one paper path is led to a return paper path 18 leading to a printing portion whereas the other paper path is led to a stacker 19.
FIG. 7 is a perspective view of the conventional switchback mechanism. One paper path is led to the printing portion by the switching gate 17 whereas the other paper path is led to the stacker 19. More specifically, through the operation of the switching gate 17, a paper sheet 20 subjected to the printing at one side thereof is fed to the return paper path 18 from its trailing end, and is again fed to the photosensitive drum 1 by the feed rolls 7, so that the printing process is repeated to thereby complete the duplex printing. The sheet 21 whose both sides have been subjected to the printing is conveyed by the conveyer belt 10 to the fixing device 11, and then is discharged by the paper discharge rollers 14.
At this time, the sheet 21 is again discharged onto the reverse plate 15, and then is again pulled by the reverse rollers 16 from its trailing end, and the switching gate 17 is operated to discharge the sheet 21 to the stacker 19. The above operation is repeated, thereby successively effecting the duplex printings of paper sheets.
However, the above-mentioned prior art has the following disadvantages:
(1) Immediately after the sheet 20 subjected to the printing at its one side is completely discharged onto the reverse plate 15 by the paper discharge rollers 14, the sheet 20, which is now free to move, tends to become slightly oblique (this phenomenon is commonly referred to as "skew") before the trailing end of the sheet 20, becomes engaged with the reverse rollers 16. As a result, the sheet is liable to become jammed in the return paper path 18.
(2) This problem, that is, the paper skew on the reverse plate 15, is liable to occur when the sheet 21 subjected to the printing at the both sides thereof is to be discharged to the stacker 19, which tends to cause the jamming in the stacker.
(3) In order to prevent the skew on the reverse plate 15, it is considered to provide fixed guides corresponding to the maximum width of the paper sheet to be used. However, when paper sheet of other sizes are to be used, the fixed guides can not be adjustably moved, and therefore the skew would also occur.
(4) In connection with the above paragraph (3), it is also considered to provide a movable guide, for example, of the sliding type. However, the operation is cumbersome, and when the operation of such a movable guide is forgotten, the jamming may occur.